1. Field of the Invention
The present invention relates to a thin-film transistor (TFT) structure and a fabrication method thereof, and more particularly, to a TFT structure and a fabrication method with a laser ablation process.
2. Description of the Prior Art
Due to the continued development in technology, flat displays have been widely used in various information products. Common flat displays comprise liquid crystal displays (LCD), organic light-emitting displays (OLED), and plasma display panels (PDP). Among the various types of flat displays, LCDs are developed maturely. Because TFT-LCDs have qualities of light weight, thinness, low energy requirements, and no radiation, they have been widely used in portable information products, such as notebook computers, personal digital assist (PDA), and mobile phones. Conventionally, the main electric elements of a LCD are thin-film transistors arranged as an array. In operation with appropriate capacitors and conducting pads, the thin-film transistors drive liquid crystal pixels to produce colorful images, and therefore a thin-film transistor is one of the key elements affecting the image quality of a TFT-LCD.
A thin-film transistor comprises a gate electrode, a source electrode, a drain electrode, and a semiconductor layer for forming a channel of the thin-film transistor. A typical fabrication process of a conventional thin-film transistor has to perform five photolithography processes, which means five photomasks is needed for defining the patterns of the thin-film transistor. However, since the costs of photomasks seriously influence the fabrication costs of display panels, a new fabrication process of thin-film transistor array by using four photomasks, including a half-tone mask, has been researched in order to reduce the fabrication costs.
With reference to FIGS. 1-4, FIGS. 1-4 are schematic diagrams of the fabrication process of a thin-film transistor by using four photomasks according to the prior art. As shown in FIG. 1, a first conductive layer and a photoresist layer are formed on the transparent substrate 10 in order. Then, a first photolithography-etching process (PEP) is carried out to form a gate electrode 12 and a wire pattern 14. Thereafter, the photoresist layer is removed. As shown in FIG. 2, an insulation layer 16, a semiconductor layer 18, an ohmic contact layer 20, a second conductive layer 22, and a photoresist layer 24 are sequentially formed on the surface of the transparent substrate 10.
Then, as shown in FIG. 3, a half-tone mask 26 is used to perform a second PEP for patterning the photoresist layer 24, wherein the half-tone region 26a of the half-tone mask 26 corresponds to the predetermined channel region above the gate electrode 12. Accordingly, an etching mask is formed with the photoresist layer 24 above the transparent substrate 10 where is a predetermined semiconductor island area. Referring to FIG. 4, the etching mask is used to perform an etching process for removing portions of the semiconductor layer 18, the ohmic contact layer 20, and the second conductive layer 22 so as to from a semiconductor island 32, a drain electrode 28, and a source electrode 30. Finally, several deposition processes, a third and a fourth photolithography step and several etching processes are carried out to form a passivation layer and a pixel electrode electrically connected to the source electrode 30 on the transparent substrate 10 so that the fabrication of the thin-film transistor and pixel electrode of each pixel or sub-pixel is finished.
As mentioned above, the prior-art fabrication method of thin-film transistors uses the half-tone mask during the second PEP process by taking its half-tone region to define the channel pattern of the thin-film transistor. Because the size of the channel pattern of the thin-film transistor is very detailed and minute, the half-tone mask for defining the channel pattern by its half-tone region has to be very accurate, whose formation cost is very high and is twice as the formation cost of normal photomask. In addition, once a defect of the transference of the channel pattern occurs during the second PEP by using a half-tone mask, it will seriously affect the electric property of the thin-film transistor, which is hard to be repaired. Furthermore, according to the prior-art process of fabricating a thin-film transistor, both of the source and drain patterns cover the semiconductor layer so that photo current is easily induced since most materials of the semiconductor layer are amorphous silicon materials that have photo sensitivity, and photo current affects the electrical performance of the thin-film transistor.
Therefore, how to fabricate thin-film transistors with good qualities by low-cost processes to avoid the photo current problem is still an important issue for the manufactures.